The present invention relates to gerotor gear sets, and more particularly, to methods of selecting design parameters of gerotor gear sets.
As is well known to those skilled in the art, a gerotor gear set of the type to which the present invention relates includes an inner rotor having a plurality N of external teeth, and an outer rotor having a plurality N+1 of internal teeth. The axis of the inner rotor is offset from the axis of the outer rotor by an eccentricity E. Relative orbital and rotational motion of the inner and outer rotors defines a plurality of expanding and contracting volume chambers, whereby gerotor gear sets are useful as fluid displacement devices. Typical examples of commercial uses of gerotor gear sets are in low-speed, high-torque hydraulic motors, hydraulic pumps, and hydrostatic steering control valves.
The term "gerotor" is a shorthand expression for the phrase "GEnerated ROTOR", because one member of the gerotor gear set includes a set of teeth or lobes which are conventionally circular, and the other member of the gear set has a profile which may be considered as having been "generated" by the lobes of the first member.
One important aspect of the design of a gerotor gear set is the mathematical relationship (clearance or interference) between the adjacent lobes or teeth of the two members (rotors). Although the present invention is not limited to any particular application of the gerotor gear set, it is important to recognize that the eventual application of the gerotor gear set, and the fluid pressure to which it is subjected, will determine the nominal, radial tip clearance between the external teeth of the inner rotor and the internal teeth of the outer rotor. For example, when a gerotor gear set is used in a hydrostatic power steering unit, there must be a positive clearance between the inner and outer rotor so that there is no binding of the rotors as the vehicle operator rotates the vehicle steering wheel. On the other hand, when a gerotor gear set is being used in a low-speed, high-torque hydraulic motor, subject to a pressure differential of 2,000 or 3,000 psi, or more, the gerotor gear set must be designed to have a nominal, radial tip clearance which is at least mathematically an interference fit, to compensate for the effect of pressure on the outer rotor, so that there is still the proper sealing between high-pressure volume chambers and low-pressure volume chambers.
A gerotor gear set has two important sealing points, which occur alternately, during relative orbital and rotational motion of the rotors. The first is a single point clearance (SPC), and the second is a double point clearance (DPC). In the case of a gerotor gear set which must not have any interference or binding, if the design values of SPC and DPC are different, the predicted manufacturing tolerance would make it necessary to select a tip clearance larger than is actually desired, in order to prevent any possibility of radial interference. The result is that the gerotor gear set will have a lower volumetric efficiency than if the design values of SPC and DPC had been the same. Similarly, if the application for the gerotor gear set requires a certain minimum interference fit, the design error or difference between the design SPC and DPC will make it necessary to increase the maximum interference fit (decrease the tip clearance further in the negative direction). This will result in a decreased mechanical efficiency.